1. Field of the Invention
The present invention relates to a variable dispersion compensator to be used for optical communication systems and particularly to a variable dispersion compensator which can vary an amount of dispersion compensation.
2. Description of the Related Art
In recent years, the requirement in the capacity for transmission of information has increased remarkably and the formation of large capacity and flexible networks is in particular demand in the optical transmission system used for the main link system. In order to cover the demand for such an optical transmission system, the Wavelength Division Multiplex (WDM) communication technique has been widely used. In WDM transmission, transmission capacity is increased by multiplexing the light beams of a plurality of different wavelengths and simultaneously transmitting a plurality of signals with only one optical fiber.
When a signal beam is transmitted with an optical fiber, the wavelength is dispersed and distortion is generated in the signal beam because group velocity of light beams is different depending on their wavelengths. As the transmission distance becomes longer, distortion of signal beam waveform increases due to the accumulation of dispersion of the wavelengths of a signal beam. When the distortion becomes large, a reception error is generated and thereby the transmission quality of signal is deteriorated. Accordingly, a dispersion compensator is used to realize long distance transmission. In this case, dispersion of the wavelengths of a signal beam is compensated for by providing chromatic dispersion of inverse sign to that of the transmission line to the signal beam in a repeating station.
One type of dispersion compensator utilizes a dispersion compensation fiber (DCF) which has chromatic dispersion of inverse sign to that of the transmission line. The DCF and a compensator utilizing fiber grating are known as a dispersion compensation device.
When the number of wavelengths of signal beams to be wavelength-multiplexed is increased to increase transmission capacity, the signal wavelength band used becomes wide. In general, the dispersion characteristic of optical fiber includes a property which depends on wavelength, which is called dispersion slope and which linearly increases depending on the wavelength. In WDM transmission, since chromatic dispersion of each wavelength of the multiplexed signal beams must be compensated, the chromatic dispersion must be compensated for the entire band of signals. Therefore, a dispersion compensator is required to provide not only an amount of chromatic dispersion compensation, but also to provide a dispersion slope, both which are equal to but of inverse sign to the chromatic dispersion characteristics of the transmission line.
A dispersion compensator preferably provides a dispersion compensation amount that cancels the amount of dispersion in combination with the chromatic dispersion of an input signal beam. The U.S. Pat. No. 2001/0024306 discloses a dispersion compensator which can vary the dispersion compensation amount by switching a plurality of dispersion compensation modules in which dispersion amounts are fixed.
For compensation of chromatic dispersion in a transmission line with a dispersion compensator, a dispersion compensation device having a chromatic dispersion of the same absolute value as that of the transmission line and the dispersion slope is necessary. In order to compensate for chromatic dispersions of the transmission lines at different transmission distances and in different installation conditions, it is required to provide dispersion compensators having various dispersion characteristics considering the range of the desired transmission distance.
In addition, since a dispersion compensation device such as DCF or the like shows considerable fluctuation in the amount of chromatic dispersion compensation and dispersion slope due to the manufacturing method, it is difficult to provide a dispersion compensation device having the desired amount of dispersion compensation and dispersion slope.
Chromatic dispersion and dispersion slope of a transmission line can be compensated for by utilizing a dispersion compensator that can vary the amount of dispersion compensation by switching a plurality of dispersion compensation modules, but a plurality of dispersion compensation devices must be provided, resulting in increase in the number of components and rise of manufacturing cost.